I learned this the hard way with my serpentine belt. WD-40 dries out rubber.

Mineral spirits with some cheap oil

Quote:

Formulation

WD-40's formula is a trade secret. The product is not patented to avoid completely disclosing its ingredients.[4][7] WD-40's main ingredients, according to U.S. Material Safety Data Sheet information, are:

It further lists flammability and effects to the human skin when repeatedly exposed to WD-40 as risks when using WD-40. Nitrile rubber gloves and safety glasses should be used. Water is unsuitable for extinguishing burning WD-40.